European patent EP 0 815 822 shows a stand-up wheelchair with a chassis and a stand-up frame that comprises a seat and a backrest. The stand-up frame comprises two parallelogram levers whose front ends are articulated to the chassis and are pivotable around fixed pivot points. The backrest is articulated to the rear ends of the parallelogram levers. Attaching the stand-up frame by way of two parallelogram levers has the advantage that the tilt angle of the backrest will not change when erecting the stand-up frame, thus the tilt angle that the backrest assumes relative to the horizontal in the sitting position is also present in the stand-up position.
A disadvantage of the stand-up wheelchair described is that the tilt of the backrest cannot be adjusted. However, backrest adjustability is desirable because, depending on the activity, wheelchair users would want to adjust the tilt of the backrest. When using the wheelchair for sporting activities (active riding position), for example, the backrest is tilted forward slightly so that it assumes an acute angle with the horizontal. But during other activities, the wheelchair user would want to sit in the wheelchair in a relaxed manner, and would tilt back (obtuse angle) the backrest accordingly.                1. For a stand-up wheelchair, it is important that when in the stand-up position, the sitting surface and the backrest surface are arranged essentially parallel to each other. But if the backrest and the sitting surface form an angle larger or smaller than 90 degrees in the sitting position, then the parallelism required disappears. The disadvantage for wheelchair users is that they cannot support their entire body with the stand-up frame in the stand-up position.        
The stand-up wheelchair according to EP 0 815 822 has a frame made from tubes upon which the front and rear wheels are arranged. The shaft of a leg support is guided into the frame of the chassis in a translational manner, giving the footrest stable guidance. The stand-up frame is connected to the shaft of the leg support in such a way that when erecting the stand-up frame, the footrest moves down. The disadvantage of this design is that the footrest is arranged so far in front of the sitting surface, that the wheelchair user is prevented from assuming a dynamic sitting position. Wheelchair users who play sport in particular want to assume an active riding position to increase the manoeuvrability of the wheelchair. This riding position is characterised in that the backrest assumes an acute angle with the sitting surface, and the feet are placed as closely as possible to the body.
DE 20 2007 018 454 U1 shows a stand-up wheelchair with a hinged parallelogram made up of two longer and two shorter levers. The longer levers are adjustable in length. The shorter and the longer levers are connected by joints with hinges that allow the levers to move only rotationally relative to each other. The rotational axis of the seat takes a different position on the frame as the rotational axis of the hinged parallelogram. This means the back adjustment has its own hinged parallelogram and the sitting surface moves independently of this. When erecting the wheelchair, the distance between the backrest and the sitting surface of the seat is reduced. It prevents a shearing motion between the back cushion and the back of the wheelchair user. However, the mechanics of this stand-up wheelchair are very complicated. The pivot points of the hinged parallelogram, the seat and the backrest are not level when in the stand-up position. As a result, the wheelchair user cannot lean their whole body on the seat and back cushion.
The stand-up wheelchair according to EP 2 389 914 A1 is constructed using a complicated lever system. One of the levers is a length-adjustable gas spring, which enables the erection of the wheelchair. The sitting surface and the gas spring have their front ends hinged to the wheelchair's holding frame and their rear ends hinged to a plate. The rear end of the sitting surface is L-shaped, thus when in the stand-up position, the rear end of the sitting surface in the area of the plate shifts backwards in the form of a step. As a result, the sitting surface and the backrest do not lie level when in the stand-up position. The plate forms an unpleasant ledge that prevents the wheelchair user from leaning against it.
An object of the present invention is therefore to provide a stand-up wheelchair that permits taking an active riding position. One object in particular is to provide a stand-up wheelchair whose backrest—despite different angular positions to the sitting surface in the sitting position—is essentially on the same level as the sitting surface in the stand-up position. The wheelchair should also allow as many settings as possible, so that the same chair can be customised to different body sizes.